One of the problems in a spread spectrum communication system using a wide frequency band consists in that communication is made impossible or the error rate is increased by a narrow band interference at a high level. In order to solve this problem, a filter using surface acoustic wave ( hereinbelow abbreviated to SAW ) has been invented.
In order to try to increase the performance of a system suitable for suppressing the narrow band interference, which is useful for the spread spectrum communication system, the inventors of the present invention have proposed a surface acoustic wave device (indicated in FIGS. 6 and 7 of the present application) in JP-A-Hei 1-267503 (which corresponds to U.S. Ser. No. 07/521,142).
In the figure, reference numeral 1 is a p' conductivity type Si monocrystal substrate; 2 is a p conductivity type Si epitaxial layer; 3 is a thermal oxide layer; 4 is a ZnO piezo-electric layer; 5, 6 and 7 are metal electrodes, which are input transducer means formed on the piezo-electric layer 4 and classifying an input signal in the frequency to generate a plurality of surface acoustic waves at respective frequencies traveling along respective propagation paths (channels), output transducer means formed on the piezo-electric layer 4, each of the output transducer means obtaining an output signal from a SAW propagated through a respective one of the propagation paths, and gate electrode means formed on the piezo-electric layer 4, each of them corresponding to a respective one of the propagation paths, respectively; 8 is a p' conductivity type high impurity concentration diffusion region formed in the epitaxial layer 2 under the input transducer means or the output transducer means; 9 is an n' conductivity type diffusion region formed in the epitaxial layer under the gate electrode; 10 is an n' conductivity type diffusion region formed in the epitaxial layer 2 outside of the input transducer means 5; and 7' is gate electrode means formed on the piezo-electric layer, corresponding to the n' conductivity type diffusion region. First pn diode array means are formed along the SAW propagation paths by the n' conductivity type diffusion region 9 and on the other hand second pn diode array means are formed along the SAW propagation paths by the n' conductivity type diffusion region 10. 11 is a resistor connected with each of the pn diode array means 9 and 10; 12 is a DC power supply; and 13 is a terminal for monitoring a voltage signal obtained by transforming an input signal into SAW detecting it by means of the second pn diode array means, by means of which terminal the intensity (electric power) of the input signal within the relevant channel (frequency region) is observed in the form of variations in the voltage. 14 is a bias control terminal for the first pn diode array means.
FIG. 8 shows a surface acoustic wave device of the above-mentioned type, which has n channels of the above-indicated SAW components connected in parallel. In particular, each of the n channels includes two diode arrays 9 and 10, an input transducer 5 between the diode arrays, and an output transducer on the opposite side of the diode array 9 from input transducer 5. Each input transducer generates surface acoustic waves at a respective frequency, which propagate along a respective one of 17 propagation paths 20.
According to the invention of the older application described above a function of monitoring the spectrum intensity of the input signal is added to the construction of a prior art element. By the prior art element only one of the signal intensities of the SAWs excited by the input transducer means and propagating in the two directions is controlled by the pn diode array means disposed on the propagation paths and the SAW propagating in the reverse direction is not used.
According to the invention of the older application the signal intensity of this SAW propagating in the reverse direction is detected by the pn diodes to obtain information on the spectrum intensity of the input signal so that the propagation control can be effected by a lower input power by means of a feedback circuit.
In the surface acoustic wave element according to the invention of the older application, contrarily to the fact that in the filter portion it is desirable that the epitaxial layer is thick in order to increase the loss of the SAW, in the detecting portion in order to increase the intensity of the detected signal, it is desirable that the epitaxial layer is thin from the point of view to decrease the loss of the SAW. However, since the invention of the older application attaches great importance to characteristics of the filter portion, the epitaxial layer is thick and as the result it has a drawback that the detected signal is small.